1. Field of the Invention
This invention relates to a method of stirring molten metal by repeatedly sucking and discharging part of the molten metal into and out of a refractory cylinder having an upper closed end and a lower opening end immersed in the molten metal, and more particularly to the refractory cylinder including evacuating and pressurizing means for carrying out the above method.
2. Description of the Prior Art
The inventors of the application have proposed a superior secondary refining method for molten steel carried out using an assembly shown in FIG. 1, wherein a lower end of a cylinder 6 made of a refractory material is immersed in a molten steel 5 in a ladle 1. The method comprises steps of evacuating the cylinder 6 to suck up an amount of the molten steel in the cylinder and pressurizing the cylinder to rapidly discharge the sucked molten steel into the ladle and periodically repeating the above operation to violently stir the molten metal in the ladle 1 with the aid of the kinetic energy transferred by the discharged molten steel. In this case, a deoxidizer or elements to be alloyed may be added into the cylinder. The added elements or the like melt into the molten steel in an inert atmosphere without any adverse effect of slag to ensure a high yield of the addition.
In order to obtain the kinetic energy for the effective stirring, the space on the meniscus of the melt in the cylinder is evacuated to raise the melt into the cylinder and then pressurized to rapidly discharging it into the ladle from the immersed end of the cylinder repeatedly at high cycles in unit time.
In actual operations on scale in the order of 300 ton, the refractory cylinder has usually 600-1,000 mm inner diameter and as much as 3,500 mm length and is provided at its upper end with pipings for pressurizing and evacuating and chutes for adding refining agents and/or elements to be alloyed.
Usually, cylindrical core metals are embedded in the immersed lower end of the cylinder for the purpose of shutting off air passing therethrough and reinforcing those areas and the upper areas thereabove are surrounded by protecting steel plates.
The larger the refractory cylinder, the greater is usually the stirring force, but the shorter is its service life. From results of experiments by the inventors, it has been found that the life of the refractory cylinder depends not only upon its size but also upon the amount of the molten steel to be treated in view of the stirring effect.
In raising and lowering the molten steel in the above manner, the inner surfaces of the refractory cylinder are necessarily damaged by the violent friction force between the moving molten metal column and the inner surfaces of the refractory cylinder at high temperatures. Also deposited solidified steel on the upper part of the inner surfaces due to splashes of the molten steel caused by the variation in pressure unavoidably lowers the performance of the apparatus.
A gas inlet usually opens into an axial direction of the refractory cylinder through a ceiling wall thereof for introducing the pressurized gas. In this case, the gas impinges directly against the uppermost surface of the raised molten metal column in the refractory cylinder so as to cause surges of the molten metal which also cause the splashes of the molten metal. Moreover, there is a tendency for the pressurized gas to be partially absorbed in the molten metal. In order to avoid adverse effect of the absorbed gas on the refining operation, a rare gas such as high purity argon is needed, which necessarily increases the refining cost because the inexpensive nitrogen gas cannot be used for this end.
As mentioned above, the splashes are caused by the surges of the molten metal which in turn limit the suction speed of the molten metal. On the other hand, higher pressure is needed for rapid discharge. However, there is a risk of violently blowing away the molten metal by the pressurized gas jetted from the immersed end into the molten metal in the event that the pressure is too high. To avoid this, if the pressure is maintained at a low value, the required stirring cannot be obtained.
By reducing the pressure to a value lower than the static pressure of the molten metal at the lower end of the refractory cylinder immediately before the surface of the pressurized molten metal in the refractory cylinder reaches the lower end thereof, the above blowing away of the molten metal may be avoided even for the rapid discharge under high pressurized pressure. However, the pressurizing period is in fact very short within at the most 0.4-0.5 sec during which the surface of the molten metal would reach the lower end thereof, so that the operation of measuring the pressure at that moment and stopping the pressurization or exhausting the pressure cannot be effected in time for the excess lowering of the surface of the molten metal because an operation of a valve takes about 0.2 sec. Up to the present, accordingly, it has been obliged to take means insufficient for stirring, such as lowest possible pressurizing speed or stable pressure range for avoiding the above precaution.
However, it is of course essential to discharge the raised molten metal in the refractory cylinder at a highest speed in order to obtain sufficient stirring energy. In this case, if the pressurizing stroke or period is delayed, the pressurizing effect will be preceded by the fall of the molten metal column due to its weight of gravity so as to decrease the accelerating effect of the pressurizing operation acting upon the molten metal column. In case of acceleration by gas pressure, the work done by the gas in expanding becomes inherently the accelerating energy, so that it is desirable for the gas pressure for pressurizing the molten metal to be as high as possible.